Cut an onion and your eyes may start to burn and tears may run down your face. Scientists have been investigating why this happens for decades. Now some researchers have uncovered a new piece of the complex answer. They have figured out how one of the veggie’s eye-irritating compounds forms.
For a common vegetable, the chemistry of onions is surprisingly perplexing, says Eric Block. He wasn’t involved with the new study. Still, this chemist at the University at Albany in New York, does know a lot about onion chemistry.
Slice into it and you unleash a chain of chemical reactions, he notes. Stable molecules in the onion’s tissues transform into a volatile, sulfur-containing gas. This gas reacts with the eyes to form small amounts of sulfuric acid. Sulfuric acid can lead to itching, burning — and tears.
But the process is more complicated than that. He and other scientists have been struggling to come up with the full picture for more than 40 years. “The more compounds we discovered, the more complicated the picture became,” Block says.
Determining the chain reaction
Block has been working on the problem since the 1970s. He determined the structure of one of the onion’s eye-irritating molecules in 1979. This sulfur-containing chemical is called lachrymatory (Lak-RIM-uh-tor-ee) factor, or LF. (This comes from the Latin word lacrima, which means “tear.”)
The structure of any molecule shows the position of the different atoms that make it up. That structure also shows how the different atoms join together. “Knowing [LF's] structure gave us clues about how the molecule formed,” says Block.
He knew that LF must start off as a different molecule. That’s because it only caused a tearing of the eyes once the onion’s skin was broken. He hypothesized that a chemical reaction must take place as an onion is cut. Block thought this chemical reaction must convert a stable molecule into one that could quickly vaporize and burn the eyes.
Sometimes chemical reactions need a little help to get started. That’s where enzymes come in. “Enzymes are amazing molecules,” he explains. They are nature’s fire-starter. Enzymes help to jump-start or speed up chemical reactions.
Block focused on an enzyme known as alliinase (AL-ee-un-ace). He showed that it helped to speed up the chemical reaction that converts sulfoxides (Sul-FOX-ides) — stable starter molecules in the onions — into eye-irritating ones. Certain plants, such as garlic and onions, make alliinase. It helps make the volatile chemicals that give these foods their savory flavors and aroma.
Later, in the early 2000s, scientists at a Japanese food company discovered a second important enzyme in onions. These scientists had been looking for a way to make a tearless onion. They thought they could do so by keeping alliinase from triggering the reaction that led to sulfuric acid. But when they isolated alliinase and mixed it with the sulfoxides, nothing happened.
That’s when they discovered LF synthase. The researchers reasoned that alliinase helped to change the shape of the starter molecule so that the second enzyme could go to work on it. Yet nobody knew for sure. The structure of that LF synthase was unknown. And without that information, researchers couldn’t decipher the complete chemical reaction.
Marcin Golczak is a biochemist at Case Western Reserve University in Cleveland, Ohio. Biochemists study the molecules at work in living things. Golczak and a team of researchers wanted to know how the eye-irritating LF forms. To do that, they turned LF synthase into a crystal. Now they could use a technique called X-ray crystallography to map its structure.
First, the researchers mixed the enzyme with an acid. This caused crystals to grow in the mixture. The researchers froze the crystals in liquid nitrogen. Then they used a special camera to shoot X-ray beams at the crystals. When those beams hit the crystal, the X-rays scattered. By measuring the angles of the scattered beams, the researchers could construct a 3-dimensional picture of the enzyme.
Golczak’s team combined this 3-D picture with the previous research findings of what the enzyme did. From this, they made a model of what the enzyme looks like. “Science is a building process. You don’t just discover one thing and then instantaneously know everything about how it works. There are many layers,” Golczak explains.
Block calls the new study the next logical step in solving a longstanding question. “It’s one small but critical piece of a puzzle,” he says.
Golczak’s team described what they did in the September 15 issue of ACS Chemical Biology.
As to why onions produce the eye-irritating chemical in the first place, Block says it probably helps the plant defend itself from predators. The chemicals that hurt human eyes might also trigger pain in other organisms. This would make the plant less appealing to wildlife, insects and even some bacteria or fungi.
“It’s an intricate reaction millions of years in the making,” Block says. “You can’t help but be in awe at the beauty of nature when you start to look at what’s happening around us on the [chemical] level.”
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angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).
biology The study of living things. The scientists who study them are known as biologists.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).
chemistry The field of science that deals with the composition, structure and properties of substances and how they interact. Scientists use this knowledge to study unfamiliar substances, to reproduce large quantities of useful substances or to design and create new and useful substances. (about compounds) Chemistry also is used as a term to refer to the recipe of a compound, the way it’s produced or some of its properties. People who work in this field are known as chemists.
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
crystal (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
crystallography A field of science that studies crystals, especially their structure and composition.
defense (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)
enzymes Molecules made by living things to speed up chemical reactions.
factor Something that plays a role in a particular condition or event; a contributor.
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
information (as opposed to data) Facts provided or trends learned about something or someone, often as a result of studying data.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
liquid A material that flows freely but keeps a constant volume, like water or oil.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nitrogen A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
sulfur A chemical element with an atomic number of sixteen. Sulfur, one of the most common elements in the universe, is an essential element for life. Because sulfur and its compounds can store a lot of energy, it is present in fertilizers and many industrial chemicals.
sulfuric acid A strong acid having the chemical formula H 2 SO 4 . Used as a drain cleaner and in lead-acid car batteries, the liquid is able to burn tissues and eat through metals and even rock.
tissue Made of cells, any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
vaporize To convert from a liquid to a gas (or vapor) through the application of heat.
volatile Chemical that easily evaporates.
X-ray A type of radiation analogous to gamma rays, but having somewhat lower energy.
Journal: J.A. Silvaroli et al. Enzyme that makes you cry—crystal structure from lachrymatory factor synthase from Allium cepa. ACS Chemical Biology. September 15, 2017, p. 2296. doi: 10.1021/acschembio.7b00336.